To maximize the stability and, in some cases, speed of naval vessels, 5XXX aluminum-magnesium alloys are often used to construct ship superstructures and more recently the hull and superstructure of certain naval vessels [e.g. the Independence variant of the U.S. Navy Littoral Combat Ships]. It is a well-known fact in Naval Architecture that every pound of structure that can be eliminated above the ship's metacenter will increase a ship's stability. It is equally well-known that every pound of structure that can be eliminated from the overall ship's weight will increase the overall efficiency of the ship.
The primary metal, aluminum, offers the advantage of being lightweight. Adding magnesium to the aluminum creates an alloy that is both lightweight and strong. To understand how sensitization, intergranular corrosion (IGC), and stress corrosion cracking (SCC) impact a naval vessels constructed from 5XXX aluminum, the composition and basic material properties of 5XXX aluminum alloys need to be explained.
The primary alloying element in 5XXX aluminum (Al) alloys is magnesium (Mg). During production of the alloy, highly controlled heat treatments are used to evenly distribute magnesium (Mg) in the aluminum (Al) matrix. Different alloys in the 5XXX series contain varying amounts of Mg ranging from ˜3.5% in 5086 to ˜4% in 5083 up to ˜5% in 5456. The evenly distributed state of the Mg within the Al matrix is thermodynamically metastable and exposure to even mildly elevated temperatures for extended periods of time will cause the magnesium to form beta-phase (Mg2Al3) precipitates. The formation of these beta-phase precipitates along the grain boundaries as a connected network is called sensitization. A structure made from a 5XXX-series alloy which has been sensitized contains a connected network of beta phase precipitates along the grain boundaries.
The rate of sensitization is primarily a function of five factors: thermal exposure, alloy composition (% Mg), material temper, grain size, dislocation density and microstructure. Assuming equivalent thermal exposures, tempers, grain sizes, dislocation densities and microstructures; 5XXX Al alloys containing higher amounts of magnesium will sensitize faster than 5XXX Al alloys with lesser amounts of magnesium. For example, 5456 (˜5% Mg) will sensitize faster than an equivalent 5083 (˜4.0% Mg) sample, and 5083 will sensitize faster than an equivalent 5086 (˜3.5% Mg) sample when exposed to the same thermal conditions.
The beta-phase (Mg2Al3) precipitates contain approximately 38% Mg which is significantly higher than the Al matrix, which for Al 5456 contains only approximately 5% Mg. Elemental Mg is thermodynamically less stable and kinetically more active than elemental Al. These characteristics make Mg more susceptible to dissolution in low and neutral pH environments. The beta-phase (Mg2Al3) behaves more like Mg than Al and will dissolve rapidly in seawater environments. This difference in dissolution behavior, combined with the fact that beta-phase preferentially forms on grain boundaries during service, leads to the preferential corrosion of those grain boundaries, which is termed intergranular corrosion (IGC).
Stress corrosion cracking will occur if a specific set of material properties and environmental conditions are present. Sensitized material is one of the conditions that contributes to SCC of aluminum alloys. The sensitized material then needs to be exposed to a corrosive environment and IGC needs to initiate corrosion along grain boundaries. Lastly, a tensile stress needs to be applied to the IGC affected material to form a stress corrosion crack. It should be noted that material sensitization alone does not result in stress corrosion cracking. For example, there have been instances on flight decks of U.S. Navy Guided Missile Cruisers where flight deck material has tested as sensitized but the lack of significant tensile stresses has historically not resulted in stress corrosion cracking problems in this area. The relationship between sensitization, intergranular corrosion (IGC) and stress corrosion cracking (SCC) is illustrated in FIG. 1.
The American Society for Testing and Materials [ASTM] has a standard test to determine if an Aluminum-Magnesium alloy is sensitized. The ASTM G67 test involves destructive testing of a sample coupon obtained from the ship structure. This test is destructive, complicated, expensive, and time-consuming. Recently, other means for testing the sensitization of a Aluminum-Magnesium alloy have become available. ElectraWatch, Inc. has developed a Degree of Sensitization (DoS) Probe which is a nondestructive tool currently approved by the U.S. Navy for quantitative material assessment of the 5456 series aluminum alloys found on many Navy ships. Use of the DoS Probe to determine sensitization of ship structure made from Aluminum-Magnesium alloy is non-destructive, can be performed on the ship, is very quick and relatively inexpensive compared to ASTM G67 testing. To date the DoS Probe has been used to conduct over 4,000 measurements on various Navy ships in support of modernization, maintenance, and repair efforts.
Stress corrosion cracking [SCC] is a serious problem in ships constructed [partially or completely] from 5XXX Aluminum-Magnesium alloys. Cracks in the superstructure of a ship can seriously weaken the superstructure leading to potential structural failure with all the associated potential for disaster. It is self-evident that cracks in the hull of a ship are a disaster waiting for an opportune time to occur. In addition, sensitized 5XXX portions of a ship can cause other problems. Ship repairs and/or modifications involving welding can be problematical when dealing with sensitized Aluminum-Magnesium alloys.
The Navy has created their own guidance on the weldability of sensitized 5XXX aluminum based on ASTM G67 testing [NAVSEA, 2013]. Under the Navy's guidance, material that tests between 0 and 20 mg/cm2 [under the ASTM G67 standard] can be welded using normal aluminum welding standards; material that tests over 20 mg/cm2 must meet critical welding requirements. Material that tests between 30 and 60 mg/cm2 must have the weld treated with cold working treatments to prevent re-cracking, and material that tests over 60 mg/cm2 must be replaced.
For these reasons, it is extremely desirable to have a means for determining when ship structures made from 5XXX alloys have been sensitized. Since it takes some time for these sensitized alloys to develop intergranular corrosion (IGC), if one knows that a particular portion of the ship structure has been sensitized, it is still possible to take corrective action before serious problems caused by SCC can occur. As noted above, testing using the ASTM G67 test or the recently available DoS Probe are available but both tests require complicated equipment and a certain amount of time and expense. In addition, both tests require trained personnel for successful testing. It would be extremely desirable to have a simple, quick test for sensitization which could be administered by relatively untrained personnel.